The inadequacy of currently-available types of biological prosthesis used for the replacement of mitral and tricuspid valves is well known, and stems from two fundamental drawbacks.
The first such drawback, relating to tissue, is the lack of long-term reliability; the second, relating to shape, is lack of appropriate prosthetic geometry.
Recently-developed techniques in the fixation of biological materials and the prevention of calcification are such as ought to prolong the average life of valve prostheses. Attempts have been made to overcome the second drawback by embodying biological and mechanical valves having in common the feature of a substantially elliptical annulus.
Such prostheses offer certain advantages over standard commercially-available types, namely: (1) better adaptation of the prosthetic annulus to the anatomical annulus; (2) reduced contact with the ventricle walls and with the left ventricular tract when opening, or with the septum and right auriculoventricular bundle (His's bundle) when closing.
The objection generally voiced against biological prostheses of the bicuspid type is that they tend to induce stenosis by reason of the geometry of the cusps themselves, notwithstanding the mitigating feature of a non-circular support medium, or `stent` so-called.
Stenosis is avoided by separating and spreading apart the cusps at the commissural ends, and inhibiting reflexive movement of these two areas by means of a sub-valvular apparatus.